In a first aspect, the invention relates to a composition for treating suspension of mineral particles in water, said composition comprising an anionic water soluble polymer and a salt of Calcium and/or Magnesium. The second aspect of the invention is a method using said composition for treating said suspension, comprising contacting the composition with the suspension.
Suspensions of mineral particles in water include all types of tailings, or waste materials. The suspensions result from mineral ores processes. They are for instance industrial tailings and all mine wash and waste products resulting from exploiting mines, such as, coal mines, diamonds mines, phosphate mines, metal mines (alumina, platinum, iron, gold, copper, silver, etc. . . . ). Suspensions can also result from drilling mud or tailings derived from the treatment of oil sand. These suspensions generally comprise mineral particles such as clays, sediments, sand, metal oxides, and may contain oil mixed with water.
The treatment of such tailings and other waste material has become a technical, environmental and public policy issue.
It is common practice to use synthetic or natural polymers such as coagulants and flocculants to separate the solids from the liquid.
For a long time, and even nowadays, mineral tailings produced by physical or chemical ore treatment methods have been stored above ground in retention lagoons, ponds, dam or embankments in semi-liquid form. These large volumes of stored tailings therefore create a real hazard, notably if the dikes break.
The accidents related to ponds and dam failures occur worldwide and are unpredictable:                Europe (14%) is the second world zone on tailings dam incidents, only surpassed by the USA (43%).        All the European tailings dam failures have occurred in dams of less than 45 m high, of which one third were in dams of 20-30 m in height.        Most of these incidents are related to meteorological causes (26% to unusual rainfall and 3% to snow). Incidents due to seismic liquefaction accounts for 14% of incidents in the world.        Over 85% of the accidents occurred in active tailings dams, and 15% of the incidents were related to abandoned dams.        
The improvement of chemical and mechanical treatments of tailings is therefore a great challenge that needs to be addressed.
Various attempts were made in the past decades to increase the settling rate of the tailings in order to efficiently recycle water and reduce the volume of tailings ponds. The main physical treatments include centrifugation, filtration, electrophoresis and electro-coagulation.
On the other hand, chemical methods are emerging. They include process involving the addition of chemicals such as sodium silicate, organic flocculants, inorganic coagulants, oxidizing and reducing agents and most recently carbon dioxide.
In 1979-1980, Alsthom Atlantique and SNF (U.S. Pat. No. 4,347,140) developed a multistep flocculation system (super-flocculation) specifically designed for treating clay lagoons from phosphate production in Florida.
The treatment of suspensions was continuously studied in 1986 according to the method described in CA 1,273,888, then in 1994 in patent WO 96/05146, in 2000 in patent CA 2,407,869 and in 2004 in patent CA 2,515,581.
In patent CA 2 682 542, the process involves the addition of polymers modified by copolymerization and/or branching. Polymers having hydrophobic groups which have also been studied showed some improvement.
Pre-treatment of tailings with multivalent cations before adding a flocculant is described for example in articles “flocculation and dewatering of Alberta oil sands tailings” (S. Ramachandra Rao), and “Flocculation of the Syncrude fine tailings” (A. Sworska).
A particular attention has recently been paid to hybrid organic-inorganic PAM containing multivalent cations. An example is AI-PAM developed by Xu and coworkers in their lab and published in article “Polymer aids for settling and filtration of oil sands tailings”. Despite these polymers has been claimed to be promising for oil sands mature fine tailings (MFT) treatment, their preparation, stability and performances remain to be verified at larger scale.
The article “Effect of multivalent salts Calcium and Aluminium on the flocculation of kaolin suspension with anionic polyacrylamide” (Felicia F. Peng) shows that both Ca2+ and Al3+ affect negatively the flocculation which means that the use multivalent ions does not improve flocculation in any cases.
WO 2012/018514 describes a method for dispersing and aggregating components of mineral slurries with zeolite and a polymer obtained by polymerization of calcium diacrylate. Polymerization of this monomer is technically difficult, and flocculation efficiency is not optimum.
Despite great advances in research over the last 10 years, there is still a need to develop new and simple solutions that may enhance the speed and amount of water released from the tailings. Improvement of the physical characteristics of the produced tailings is also sought. As industrials are very concerned by simple process, it is still an objective of the present invention to find simple and industrial method to improve flocculation of tailings.